In broad terms, the overall goal of this research program is to understand the mechanisms of neurotransmission in the vertebrate retina. More specifically, this research program will provide new information about intrinsic, Ca2+-dependent mechanisms that regulate synaptic vesicle dynamics in retinal bipolar cells. These neurons provide the major conduit through which visual information is relayed across the vertebrate retina. Bipolar cells can release a brief burst of neurotransmitter, to signal contrast, and they cn release neurotransmitter continuously, to provide an on-going read-out of luminance. Yet, little is known about the molecular mechanisms that support these behaviors, particularly at the level of synaptic vesicle dynamics. This research program is designed to provide this essential information for the rod-driven bipolar cell. First, we examine two Ca2+-interacting proteins of rod bipolar cells, the plasma membrane Ca2+ ATPase, a major regulator of bipolar cell intraterminal Ca2+, and CaBP5, a Ca2+-binding protein of poorly-understood function required for normal rod-pathway signaling. Each has the potential to modulate synaptic vesicle dynamics via local modulatory actions at or near the fusion machinery. In addition, they may modulate synaptic vesicle dynamics via the regulation of bulk intraterminal Ca2+. We will use a powerful combination of biophysical, pharmacological and molecular approaches, including the use of genetically-engineered mice, to determine the roles that these proteins play in the modulation of synaptic vesicle dynamics and the determination of synaptic gain. We will employ a similarly powerful combination of techniques to examine the roles of Munc13 and downstream Ca2+ signaling pathways in synaptic vesicle priming and short-term plasticity. Results of the proposed research program will yield new, molecular-level information about intrinsic Ca2+-dependent mechanisms that support continuous release and preserve synaptic gain in the bipolar cell synaptic terminal. In addition, results of this research program will shed light on general presynaptic mechanisms that mediate short-term adaptive changes throughout the nervous system. Finally, given the central role of bipolar cells in several strategies of vision restoratio, knowledge of the molecular mechanisms underlying the intrinsic Ca2+-dependent regulation of synaptic vesicle dynamics may suggest new targets of therapeutic intervention that will help improve visual outcomes.